微生物采油技术在中国胜利油田罗801区块的试验A-英文译文2

MEOR Field test at Block Luo801of Shengli Oil Field in
China
Abstract
A Microbial Enhanced Oil Recovery (MEOR) field test has been implemented for 10 consecutive years at the 75℃-80℃Block Luo801 in Shengli Oil Field in China, where microbial formulation and nutrient, producing biosurfactant, were injected into oil wells. After the implementation of MEOR, the produced fluid samples from the oil wells were analyzed and found an increase of bacteria and archaea in waterphase stabilizing at 103-4cell/ml and a soar of acetate in concentration from 70mg/L to 523.0 mg/L without finding the type of injected microbial types, suggesting the nutrient injected has activated the indigenous bacteria in the reservoir, whose activities were enhanced to prevent the water cut from rising up in the block so as to increase the oil output. Up to December 2010, the cumulative increased production of oil has been 9.75 × 104t with the total output surpassing that of Water Flooding Oil Recovery (WFOR) by 3.35%. Considering the cost of injected microbial fluid and nutrient, oil increased per tonne costs about RMB339.56.
Keywords: Microbial Enhanced Oil Recovery (MEOR), Field Test, Water Flooding 1. Introduction
Fault block oil reservoirs occupy an important proportion in Shengli Oil Field of China, whose developed geological reserves have reached 9.22 ×108t but with the comprehensive water cut 91.5%. Thus how to stop the production decline and enhance the crude oil recovery becomes a major challenge to develop fault block oil reservoirs with high water cut. Due to the smaller area bearing oil and the incomplete production well grid, it is not suitable to carry out chemical enhanced oil recovery technology for fault block oil reservoirs and alternative oil recovery technology is expected to be developed for them. Microbial Enhanced Oil Recovery (MEOR) technology is considered promising for fault block oil reservoirs,whose related research was not launched until 1990s in Shengli Oil Field by a specialized team engaged in research and test in MEOR.Once their purpose was to select the bacteria type, suitable for the reservoir environment and capable to enhance the oil displacement efficiently, which was first ground cultured then injected into the reservoirs. At the beginning of this century, research revealed indigenous bacteria in reservoirs are very active, so the research focus shifted to indigenous bacteria enhanced oil recovery. Currently, oil reservoirs have been considered as an ecosystem into which material such as specific microbial formulation, inorganic or organic nutrient and air have been injected to regulate them: the microbial community
structure and function inside might change, increasing the bacteria density, producing biosurfactant and improving the oil-wash efficiency through "in situ breeding". Based on research in the laboratory, a MEOR field test has been carried out in Block Luo801 in Shengli Oil Field since 1999. The test method has been adjusted twice in the 10 year period in the hope of exploring the potential of MEOR technology to improve oil recovery in fault block oil reservoirs.
2. Test Area Reservoir Geology
Block Luo801 is a small block in Shengli Oil Field, a lithologic - structural reservoir with a temperature ranging from 75 ℃to 80 ℃. It is open in the north and the southern part is closed by an east-west arc fault, where the test area is located on the hanging wall. The area bearing oil is 1.95 km2 with the crude oil reserves 291 ×104t. The reservoir depth is from 1680 m to 1800 m and the original formation pressure is 17.5 MPa.
The oil pool of the test area is located on the West Wing of the nose-like structure, whose strata are high in the south and low in the north with an angle ranging from 5ºto 6º. It passes through Permian strata including Shahejie Fm., Dongying Fm., Guantao Fm. and Pingyuan Fm. as a lake phase mesoporous and meso-permeable reservoir, whose oil bearing units are S1, S3, S4 of sand section. with an average porosity 23.5%, an ⨯10-3μm2, and the ground oil viscosity 221.7mPa.s, formation water type NaHCO3 and TDS 7795 mg/L. Test area reservoir parameters are shown in Table 1 and the wells’ position is shown in Figure 1.
Figure 1 Tectonic position of MEOR test area in Luo801
3. Test Area Development Process
Lou801 has been formally developed since May 1994, in June 1998 flooding oil recovery began here, in July 1999 the first MEOR field test launched and in 2003 air aided MEOR began to develop. According to the test area production curve, the development can be divided into four stages, namely capacity building stage, natural energy development stage, flooding development stage and MEOR test stage.
3.1 Capacity Building Stage, 1994
Wells Luo801 and 805 have been finished drilling in Block Luo801 in April and May 1994 respectively. Since industrial oil flow obtained in S3 and S4, the block was under rolling development as in the same year, an oil bearing area of 2went into operation involving oil reserves 291 × 104t with a new production capability 6 × 104t. In December 1994, 16 wells were opened and produced 446t of fluid and 417t of oil per day with the water cut 6.5%, oil production rate 5.23%, recovery percentage 3.16%, outputting 9.2097 × 104t of oil annually.
3.2 Natural Energy Development Stage, 1995 –May 1998
This stage features relying on natural energy when the reservoir was developed at a high speed. With the formation pressure dropping rapidly, the production declined fast when it with a pressure drop up to 10.87MPa, less than 30% of the saturation pressure . The elastic yield is 104t/MPa. The annual output of oil decreased from the highest 11.9 × 104t to 6.3 × 104t, with the annual declining 12% . Late in this stage, 16 wells were open, producing of liquid and 195.3t of oil per day with the water cut 40.5%, whose working fluid level was 958m. In this stage, the oil output reached 30.3114 ×104t at the recovery percentage 10.4%. Cumulative oil production was 39.5211 × 104t covering 13.58% of the reserves.
3.3 Flooding Development Stage, June 1998-June 1999
Block Luo801 was developed by flooding in June 1998, first with Wells L801-7, L801-15, L801-17. At the end of this stage, 13 wells were open, producing 259.6t of liquid and 96.7t of oil per day with the water cut 62.8%. The working fluid level was 1399m. At this stage, 2.3612 × 104t of oil, 0.81% of the reserves, was obtained. The cumulative oil production reached 41.8823 × 104t, 14.39% of the reserves.
3.4 MEOR Test Stage, July 1999-December 2010
MEOR was launched in Luo801 in July 1999. In the initial phase, micro-organisms was injected into three injection wells (L801-7, L801-15, L801-17), in September 2002 the recovery pattern was adjusted to have the middle two injection wells ( L801-3, L801-18) injected with the aid of air compressors injecting air.
As of December 2010, 14 oil wells in Luo801 were put into operation, 12 were opened and producing 448.2t of liquid and of oil per day. The cumulative oil recovery was 85.6572 ×104t with the water cut 86.5%, covering 28.17% of the reserves. The oil producing rate was 0.77 %. A total of five injection wells were opened, injecting3 per day with a cumulative injection 110.023 × 104m3. The monthly injection-production ratio was and the cumulative injection-production ratio was 0.40. Two MEOR test wells were injected with 145m3 of water per day.
4. Overview of MEOR Field Test Implementation
The implementation of MEOR field test of Block Luo801 can be divided into two phases. July 1999 to September 2002 is the routine MEOR phase, when microbial liquid and nutrient were injected from the three side injection wells. The liquid contains 4 types of bacteria, producing similar substance as biosurfactant; main ingredients of the nutrient solution were sugar and a small amount of inorganic salt.
11 wells were affected in the test. By periodic injection technique, the bacterial liquid and nutrient solution were injected into the formation once every 20 days with water. September 2002 to December 2010 is the air aided MEOR phase. Air injection equipment was adopted. Microbial liquid and nutrient solution were injected into two middle wells (L801-3, L801-18) of the block, affecting 14 wells. In addition to water injection normally, air was injected 12 hours a day.
As to December 2010, 3235t of bacteria liquid, 3475t of solid nutrient and 36.41 × 104Nm3 of air were injected.
5. Analysis of Test Results
After the implementation of MEOR, the overall comprehensive water cut increasing is inhibited in the test area, the natural decline of production is slowed down and the daily oil production rises steadily. Especially after the implementation
of air-aided MEOR, the production is better than MEOR stage and apparently better than the production status before the test.
5.1 Affect of MEOR on Comprehensive Water Cut of Oil Wells in Test Area
After changing into the water injection recovery, the water cut increasing reached as high as up to 8.43%. During the implementation of MEOR, the water cut increasing significantly slowed down. The water cut increasing rate has been less than 1% for 7 years continuously, showing that the water cut increasing has been effectively suppressed.
The water cut curve in the test area shows two trends with different characteristics 5-6 months after the implementation of MEOR and air aided MEOR. Compared with the water cut curve of water flooding development, the curve of MEOR has a “funnel” shape with wider opening and shallower bottom, lasting a longer time with a slower pick up. Furthermore, with the recovery percentage, the water cut increasing rises slower. This indicates that MEOR technique, milder and effective in longer time, has better improving effect on reservoirs than chemical flooding recovery, (Figure 2).
Figure 2 Relationship between water cut and recovery percentage
5.2 Effect of MEOR on Oil Production
During the period of MEOR test in Block Luo801, no measures of significantly changing production were adopted, such as drilling new holes, drilling new wells and increasing injection, besides routine maintenance like oil well pump inspection, operation and parameter adjustment. Therefore, the result can be compared before and after the MEOR test. Based on reservoir engineering methods, the production decline of the test area consists with the hyperbolic diminishing law. As of December 2010, cumulatively an additional 9.75 × 104t of oil were produced, and the total product is 3.35% more than that of flooding oil recovery, showing MEOR development is obviously better than flooding recovery in the same type of reservoirs (Figure 3).
Figure 3 Actual production and decline forecast curves of Luo 801
5.3 Development Effect Improvement
1) Slow the natural decline
After the implementation of MEOR, the production decline has slowed down significantly in Luo801 with the highest natural decline rate from 32% to -2.3% in 2005. Though production has an overall downward trend, the average decline rate is
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2) Enhance oil recovery
After the implementation of flooding and MEOR, the situation has been improved in Luo801. The recovery rate rises obviously, which was 38.02% based on Flooding Dynamic Calibration, increasing by 9.42% over the one calculated with the empirical formula of Shengli Oilfield, 28.6%. If the MEOR operation goes on under the current conditions, the ultimate oil recovery rate is expected to increase by 10.17% to achieve extra recoverable reserves 29.6 104 t.
5.4 Extend Oil Pump Inspection Cycle
Statistics of the oil pump inspection cycle before and after MEOR show that the inspection cycle is prolonged tremendously, 9.7 months vs 21.5 months, 11.8 months outreached. Analysis reveals that micro-organisms and their metabolic production migrated to oil wells to lower the speed of waxing and to maintain the shaft.
5.5 Better than Flooding Recovery of Same Type Fault Block Oil Reservoir
Blocks Luo9 and Luo801 of Luojia Oilfield are of the same type development units with similar reservoir conditions. Luo9 is situated to the south of Luo801, its main oil-bearing stratum is S1 of sand section. It was put into operation in 1989, and switched to flooding recovery in 1990 (Table 2). The parameters of the two blocks are listed in the following table.
By comparison, it is clear that the nature of oil from Luo9 is better with the ground oil viscosity 16.8MPa.s, formation oil viscosity 6.5mPa.s, sulfur 0.8%, wax 17.1%, freezing point 29℃and its water and oil ratio is lower than that of Luo801. It was water injected during the initial stage so as to compensate the formation energy in time, ensuring the flooding development effect. Its benchmark flooding recovery is 34.1%. The reservoir temperature of Block Luo801 is ranging from 75℃to 80℃, the viscosity of crude oil is relatively high with the ground oil viscosity 221.7mPa.s, formation crude oil and wax 3.7%. It has been degassed by dissolved gas during the stage of natural energy development. At the pre-MEOR stage, its ground viscosity of crude oil reached 221.7mPa.s, the viscosity difference between water and oil was relatively large. The water cut and recovery percentage curves of the two blocks show when recovery percentage is greater than 20%, Luo801 curve extends downward to form a “funnel”. The development data of Luo801after adopting MEOR can establish the recovery of Luo801 as 38.0%, better than flood development recovery in Luo9. This shows that MEOR effect is better than that of water flooding (Figure 5).
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Figure 5 Relationship between water cut and recovery percentage curves
5.6 Characteristics of Microbial Activity in Reservoirs
During the test, microorganisms and their metabolic production of the produced fluid have been sampled and analyzed regularly. The analysis result shows that after implementing MEOR, the bacterial density increases significantly in the produced fluid, gradually stabilizing at a magnitude 103-4 / ml, while density of the sample before the test is only below 10 / ml. Analysis on concentration of acetate in the produced fluid discloses that average acetate concentration from the test area is 70mg/L before the test, which changes to mg/L -mg/L, averaging 523.0 mg/L, after the implementation of MEOR(Figure 6). During the test, the composition of produced gas was analyzed and the relative concentration of methane gas was found to increase (Figure 7). The researchers believe that as the anaerobic anaerobicbacteria in the reservoir is activated, part of the acetate is utilized by anaerobic microorganisms to have the acetate concentration decrease. Monitoring on the methanobacterium content in the produced fluid reveals that the number of total bacteria and methanogenic bacteria levels continue to rise, which might be caused by acetate-like metabolic production migrating to deep of the oil reservoir and being utilized by methanogens in anaerobic environment.
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时间（年月）A c -含量m g /L L801-1801-5801-6801-X9801-14注入水
Figure 6 Water injection and acetate content curves in produced fluid of Luo801
Figure 7 Detection of methane gas in Luo801
Electrophoresis analysis of TRFLP and DGGE shows that, microbial species and number are very abundant in Block Luo801, with widespread archaea flora such as Thermodesulfovibrio and Methanobacterium , bacteria such as Pseudomonas and Bacillus , which have adapted to the reservoir conditions and whose ecological structure is gradually stabilizing.
6. Conclusions
MEOR technology has improved development in Block Luo801, but not as the chemical flooding technique to have significantly and substantially increased the production. Production by MEOR increases mildly but remains effective in longer time, which might be a feature of MEOR.
Analysis on produced liquid samples from the oil wells show that during the implementation of MEOR, the acetate content increases significantly and maintains at high concentration levels, suggesting a strong microbial metabolism in the reservoir. However, the bacteria types injected were not found in the produced liquid, only to find some common bacteria and archaea flora in reservoirs, which is speculated that the bacteria injected were absorbed into the porous media in the reservoir, or simply the bacteria could not survive in the reservoir environment, and the microbial activity in the reservoir is of the indigenous bacteria activated by injected nutrient.
As of the end of December 2010, the cumulative 3235t of microbial fluid and 3475t of nutrient were injected in the test area with an output of additional 9.75 × 104t crude oil. The cost of each incremental tonne of oil is .。